啊
a
Sentence-final interjection "a"
啊 warms up whatever it's attached to — agreement, surprise, or a friendlier question.
Sentence + 啊
How it works
是啊！ is an enthusiastic "yeah!". 谁啊? is "who is it?" asked casually rather than sharply.
啊 carries no meaning of its own; it adjusts tone. Leaving it out makes speech sound clipped, which is why beginners often come across as blunter than they intend.
Examples
shì a!
是啊！
Yeah, exactly!
shéi a?
谁啊？
Who is it?
hǎo a, wǒ men qù ba
好啊，我们去吧。
Sure, let's go.